Fixation systems can be used in orthopedic surgery to maintain a desired spatial relationship between multiple bones or bone fragments. For example, various conditions of the spine, such as fractures, deformities, and degenerative disorders, can be treated by attaching a spinal fixation system to one or more vertebrae. Such systems typically include a spinal fixation element, such as a rigid or flexible rod or plate, that is coupled to the vertebrae by attaching the element to various anchoring devices, such as screws, hooks, or wires. Once installed, the fixation system holds the vertebrae in a desired position until healing or spinal fusion can occur, or for some other period of time.
Placing the fixation system typically requires that one or more rods be bent or contoured very accurately to fit within several bone anchors implanted along the spine. The rods are typically bent in three dimensions and may need to be bent to match adjacent rods that are working in concert, to accommodate a deformity, or to account for gravity and soft tissue forces. In addition, contouring a rod too many times can reduce the fatigue life of the rod. In view of these and other challenges, the precise contouring of a rod to approximate the rod to a series of bone anchors is a time consuming and artful procedure.
It can also be a very important procedure. The alignment of a patient's vertebrae is usually a key output of the surgery and the patient's wellbeing can depend on post-operative alignment. The rods apply the corrective loads to the spine and are what hold the alignment post-surgery while healing or fusion takes place. If the rods are not contoured accurately, too much pre-stress can be applied to the bone anchors, which can result in mal-union or mal-alignment. For example, when a rod is improperly contoured, significant reduction forces must typically be applied to bring the rod into alignment with the bone anchors. These forces can cause unequal loading or loosening of the bone anchors at the anchor-bone interface, which can result in instrument or construct failure, non-union, or pain. As shown in FIG. 1A, application of significant reduction forces, e.g., using a reduction instrument of the type shown in FIG. 1B, to bone anchors at intermediate levels of a fixation construct can cause damage to the osseous threads and loosening or dislocation of the bone anchors. A similar phenomenon can occur at the end levels of the construct. For example, performing a cantilever rod reduction maneuver to restore lordosis (as shown in FIG. 1C) can result in significant reduction and recoil forces at the screw-bone interfaces (as shown in FIG. 1D), which ultimately can lead to failure at the anchor-bone interface with loosening or dislocation of the bone anchor (as shown in FIG. 1E).
Application of significant reduction forces can also cause iatrogenic central or foraminal stenosis by segmental translation of one vertebra in relation to another. For example, as shown in FIGS. 1F-1G, rod-to-anchor reduction can cause sagittal plane translation of one vertebra (e.g., C4 as shown) relative to an adjacent vertebra (e.g., C5 as shown), pinching the nerve tissue and resulting in pain, weakness, numbness, loss of function, or other symptoms.
Some surgeons may try to limit the reduction forces by backing out the bone anchor a few turns to meet the rod, however this can have the same effect of compromising bone purchase and weakening the anchor-bone interface.